Why use a resistor when blocking DC at the output of an analog source?

[ygg-it]

Think when you connect your Cd Player or a Preamplifier to your amplifier via RCA cables.


Why use a resistor R1 when blocking DC at the source analog at the Cd player or preamplifier output?

Can R2 of the next stage (amplifier) works as well for source ground reference?

I tested without R1 and it works fine, I'd say better...

Is it really mandatory R1?

FYI: Cout is very high...several uF

.....Cout........
----||-------out (coax cable)
.............|....
.............R1..
.............|....
............///...
..................


.......Cin.......
in----||-----
.............|....
.............R2..
.............|....
............///...
..................

 

[tomchr]

If the capacitor is floating charge will accumulate on it. Who knows where the final voltage ends up being. Thus, it's not a good practice to leave capacitors floating like you suggest. You should also ground the input pin with a resistor 10*R2.

Tom

 

[Gnobuddy]

Can R2 of the next stage (amplifier) works as well for source ground reference?

Yes, but there may be a huge thump or pop when the connection is made, if both devices are already powered up. The thump/pop happens because Cout has to charge or discharge to it's equilibrium state through R2.

The thump will be more severe and longer lasting if Cout is very large. Depending on the type of amp you have, the thump/pop could even be damaging to your speakers or dangerous to your hearing.

If the connection is made permanently, you may not need R1. If, however, there is switching involved - say, you switch between a CD player and a computer playing FLAC files - then the thump/pop can potentially happen any time you make the switch. In this sort of situation, I would say it's much better to include R1.

The presence of R1 will not change the sound, unless you make a mistake choosing the values of R1 and Cout. Just make sure their 3dB frequency is below the audio range, and there will be no change whatsoever in sound due to R1.

-Gnobuddy

 

[abraxalito]

One reason I can think of is if you power up the source with nothing downstream connected then the output voltage is going to still be zero. Without the resistor if you connect the destination equipment with power already applied there will be a loud 'thunk' while the cap charges.

I think R1=100k is too low a value myself, should be closer to 1M.

 

[DF96]

If Cout is an electrolytic then R1=100k may be slightly too high a value.